Was looking at Nautilus this weekend - the ones we have now are in my parents' and baby-sitter's cars so I don't see them all the time. They didn't seem that wide. @XanaduMama, I believe you own a Nautilus and Radian. Can you (whenever you get a chance) compare the width of those two seats IRL?
I measured my Radian (65, which apparently is the same size as the new R100) and Nautilus this morning. I was in a bit of a rush, so the figures may not be precise, but they should give you a general idea. The Nautilus was about 19.5" at its widest point (across the lap/armrests); the Radian was about 17-18". So not all that much difference! I was surprised. hth! Let me know if you'd like any other details, or photos or whatever--I'd be glad to help.